Typically systems that might potentially use optical interconnects within the next few years are servers, supercomputers and telecom switch-routers, comprising multiple racks, wherein each rack has a backplane or midplane, and multiple plug-in cards (or blades). Communication between the plug-in-cards is routed through the backplane. The aggregate data rate within a single rack is expected to reach multiple Terabit per second (Tb/s) by the end of the decade, and it is assumed that it will grow to tens of Tb/s in the future. Currently communication is achieved electrically through copper lines. However, in electrical lines the attenuation and cross-talk increase with frequency. For board to board interconnects with a typical distance of one meter the crossover point where optical transmission becomes more power efficient than electrical transmission is at 5 Gb/s. Furthermore, optical transmission lines can be packed much denser than electrical lines.
The reliable, accurate and cost-effective integration of optical elements such as waveguides, transmitters and detectors in PCB's is a challenge lying mainly in high integration and optical alignment. The alignment requirements of optical elements are much tighter than those of electrical components. For example, the minimum alignment tolerance in the realization of a printed circuit board is 20 μm; optical elements need to be positioned with an accuracy of 5 μm (for 50 μm×50 μm square optical waveguides). Consequently, it is a challenge to align optical elements in a printed circuit board.